Field of the Invention
The present invention generally relates to a method and apparatus for transmitting and receiving data in a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM). More particularly, the present invention relates to a method and apparatus for efficiently transmitting and receiving channel data for which a coding process is not required.
Description of the Related Art
Recently, a large amount of research is being conducted on an Orthogonal Frequency Division Multiplexing (OFDM) transmission method serving as a scheme useful for high-speed data transmission using a radio channel in a mobile communication system. The OFDM scheme is a type of Multi-Carrier Modulation (MCM) scheme for converting a serially input symbol stream in parallel and then modulating and transmitting parallel signals through a plurality of orthogonal subcarriers, in other words a plurality of subcarrier channels. The OFDM transmission scheme copies a second half part of an OFDM symbol, attaches the copied part as a Cyclic Prefix (CP) before the OFDM symbol, and transmits the OFDM symbol, thereby removing InterSymbol Interference (ISI) from a previous symbol. The OFDM transmission scheme, robust to a multipath fading channel, is suitable for broadband high-speed communication.
FIG. 1 is a block diagram illustrating a structure of a conventional transmitter in a wireless communication system based on OFDM.
Referring to FIG. 1, a channel encoder 101 receives a predetermined information bit stream and then performs a channel coding process for the received information bit stream. Conventionally, the channel encoder 101 can use a convolutional encoder, a turbo encoder, a Low Density Parity Check (LDPC) encoder, and so on. The encoded information bit stream from the channel encoder 101 is input to a modulator 103. The modulator 103 modulates the encoded information bit stream in a predefined modulation scheme such as Quadrature Phase Shift Keying (QPSK), 8-Phase Shift Keying (8PSK), 16-Quadrature Amplitude Modulation (16QAM), and so on. It is obvious that a rate matcher (not illustrated) for performing repetition and puncturing functions and so on may be additionally inserted between the channel encoder 101 and the modulator 103.
A Serial to Parallel Converter (SPC) converts an output signal of the modulator 103 to parallel signals, and then inputs the parallel signals to an Inverse Fast Fourier Transform (IFFT) processor 107. The IFFT processor 107 transforms the parallel signals according to an IFFT. A Parallel to Serial Converter (PSC) 109 converts the transformed parallel signals to a serial signal. A CP inserter 111 inserts a CP for preventing interference into the serial signal (or symbol), and then outputs the symbol with the CP. An OFDM symbol stream into which the CP has been inserted is transmitted to a wireless network through a Radio Frequency (RF) processor 113 and an antenna 115.
When the above-described conventional OFDM transmitter performs a transmission operation, a modified multiplexing scheme performs a Hadamard transform on modulated symbols to be transmitted from the OFDM transmitter in a frequency domain and transmits the transformed symbols without directly transmitting one modulated symbol through one subcarrier. This scheme is referred to as Multi-Carrier Code Division Multiplexing (MC-CDM) or Orthogonal Frequency Code Division Multiplexing (OFCDM). Hereinafter, MC-CDM and OFCDM are referred to as the OFCDM scheme.
FIG. 2 is a block diagram illustrating a structure of a conventional OFCDM transmitter in the wireless communication system based on OFDM. The OFCDM transmitter of FIG. 2 is configured by adding a well-known Hadamard transform processor 210 to the OFDM transmitter of FIG. 1 such that a CDM transmission scheme is applied to the OFDM transmission scheme.
Referring to FIG. 2, a channel encoder 201 receives a predetermined information bit stream and performs a conventional channel coding process such as convolutional coding, turbo coding, Low Density Parity Check (LDPC) coding, and so on. The encoded information bit stream from the channel encoder 201 is input to a modulator 203. The modulator 203 modulates the encoded information bit stream in a predefined modulation scheme. A Demultiplexer (DEMUX) 205 of the Hadamard transform processor 210 demultiplexes the modulated signal (or symbol stream) into N outputs. A plurality of covers with Walsh functions, in other words Walsh covers 0˜N, 207 cover the N output signals with predefined Walsh codes. An adder 209 computes a sum of the signals covered with the Walsh codes, and outputs the signal sum to an SPC 211. An output of the SPC 211 is transmitted to a wireless network through an IFFT processor 213, a PSC 215, a CP inserter 217, an RF processor 219, and an antenna 221.
In the above-mentioned two multiplexing transmission techniques, in other words the OFDM and OFCDM schemes, one scheme does not always outperform the other scheme. Relative performances of the OFDM and OFCDM schemes can differ according to many factors. The main factors capable of varying the performances of the OFDM and OFCDM schemes are the code rate of transmitted data, channel frequency selectivity, and so on. As described above, simulation results of a performance comparison between the OFDM and OFCDM schemes according to a code rate, channel frequency selectivity, and so on are illustrated in FIGS. 3 to 5. In FIGS. 3 to 5, the horizontal axis represents a signal to noise ratio (Eb/Nt) when transmitted data is received, the vertical axis represents a Packet Error Rate (PER), EG represents equal gain paths, and UEG represents unequal gain paths.
FIGS. 3 to 5 illustrate results of a performance comparison between the OFDM and OFCDM schemes, for example, when code rates of transmitted data are ¼, ½, and ⅘, respectively. It can be seen that the OFDM scheme outperforms an OFCDM (or MC-CDM) scheme when the code rates of transmitted data are low (¼ and ½) as illustrated in FIGS. 3 and 4. Moreover, it can be seen that a performance differs according to the number of equal/unequal gain paths even when frequency selectivity is varied. As illustrated in FIG. 5, it can be seen that the OFCDM scheme outperforms the OFDM scheme when the code rate of transmitted data is high (⅘).
Because performance differs according to a code rate or coding of a transmitted channel in the wireless communication system based on OFDM, there exists a need for a method apparatus, and system for efficiently transmitting data while considering this difference.